Light emitting diodes (LEDs) have been used for decades in applications requiring relatively low-energy indicator lamps, numerical readouts, and the like. In recent years, however, the brightness and power of individual LEDs has increased substantially, resulting in the availability of 1 watt and 5 watt devices.
While small, LEDs exhibit a high efficacy and life expectancy as compared to traditional lighting products. A typical incandescent bulb has an efficacy of 10 to 12 lumens per watt, and lasts for about 1,000 to 2,000 hours; a general fluorescent bulb has an efficacy of 40 to 80 lumens per watt, and lasts for 10,000 to 20,000 hours; a typical halogen bulb has an efficacy of 20 lumens and lasts for 2,000 to 3,000 hours. In contrast, red-orange LEDs can emit 55 lumens per watt with a life-expectancy of about 100,000 hours.
When manufacturing LED light engines, it is important that the device outputs light energy having an appropriate color rendering index (CRI). The CRI of a light source provides an objective measure of how particular colors will look when illuminated by the light source. Unfortunately, because the CRI is only determined by reviewing how a small number of colors are illuminated, the metric is often a poor measure of perceived lighting quality. In fact, many illumination engineering societies around the world, as well as the US Department of Energy, recognize that a new CRI metric should be developed for correctly measuring a color rendering index of a light source. However, before such a new metric can be developed, the current CRI metric is still the standard for measuring the color rendering of light sources. Accordingly, in most commercial environments, it is necessary for a particular light source to provide a high CRI in order to be commercially competitive.
Using LEDs, it is difficult to manufacture light engines having commercially attractive CRIs. For example, in a light source having a red, green and blue (RGB) mixed LED light engine, it is difficult to generate a high CRI. Accordingly, even though the LED light engine may have attractive light-generation properties, it is assigned a low CRI that does not indicate the true color rendering or the true visual performance of the RGB LED light engine. The reason for the low CRI from a RGB mixed white light engine is the absence of the yellow spectrum having wavelengths of 560 nm to 580 nm. Unfortunately both of the widely used semiconductor compounds AlGaInP and InGaN are not efficient when emitting light around the yellow spectrum. Although an amber 589 nm LED may be added to the RGB light engine to boost the CRI of the light engine around the yellow spectrum, the inefficiency of amber LEDs will reduce the overall lumen output of the LED light engine. For general illumination lighting devices this is not an attractive option because the LED light engine may be required to pass Energy Star V 1.0, which mandates the device efficacy exceed 35 lumens per watt, for example.
In application, the CRI is composed of 14 color charts: R1, R2, R3 to R14. All 14 color elements must be scored well in order to get a high CRI. This requires that all visible band spectrum from 400 nm to 700 nm be present in a white light. The incandescent white light spectrum, which is shown in FIG. 1 demonstrates the full visible band spectrum and a CRI score near 100. With reference to FIG. 1, an absence of 560 nm to 580 nm, as is found in most LED light engines, will jeopardize the CRI score.